In a typical cellular wireless communication system, an area is divided geographically into a number of wireless coverage areas (e.g., cells and cell sectors), each defined by a radio frequency (RF) radiation pattern from a respective base station. One or more base stations of the system may then be coupled with other network infrastructure, such as a base station controller, switch, and/or gateway, which may control aspects of base station operation and may provide connectivity with one or more transport networks such as the Public Switched Telephone Network and/or the Internet for instance. When a mobile station, such as a cellular telephone, personal digital assistant, pager, or appropriately equipped computer, for instance, is positioned in a wireless coverage area, the mobile station may then communicate via an RF air interface with the serving base station and thereby establish communication with other served mobile stations or with entities on one or more connected transport networks.
In general, the air interface in each wireless coverage area of such a system is divided into a number of channels for carrying bearer traffic (e.g., voice and other user data) and control data between the base station and served mobile stations. The channels that carry data from the base station to served mobile stations are considered “forward link” channels, and the channels that carry data from served mobile stations to the base station are considered “reverse link” channels. Depending on the wireless technology used, the air interface can be divided into these channels through code division multiplexing (with each channel defined by modulation with a specific code), time division multiplexing (with each channel defined as a segment of time), frequency division multiplexing (with each channel defined by modulation with a specific frequency), and/or some other mechanism.
In terms of control channels, the forward link of each wireless coverage area may define one or more overhead channels through which the base station may generally broadcast system parameter information for use by served mobile stations, and the forward link may further define one or more paging channels through which the base station may transmit control channel messages typically targeted to specific mobile stations. In turn, the reverse link of each wireless coverage area may define one or more access channels through which served mobile stations can transmit to the base station requests to initiate calls (e.g., voice calls or data communication sessions) and responses to control channel messages received from the base station.
The wireless coverage areas (e.g., cell sectors) in a typical system may be grouped into paging zones to help the system determine where to page a mobile station. An overhead message broadcast in each coverage area may specify the coverage area's paging zone, so that served mobile stations can detect when they move from one paging zone to another. In practice, when a mobile station initially powers on in a given coverage area, the mobile station may register with the network by transmitting a registration message via an access channel to the network, so that the network will know the paging zone in which the mobile station is located. In turn, when the mobile station moves into a new paging zone, the mobile station may then newly register with the network, to notify the network of the mobile station's new paging zone. When the network seeks to page the mobile station, the network may then transmit the page message in all coverage areas of the mobile station's zone of last registration, and perhaps in adjacent zones to account for the possibility that the mobile station has moved to a new zone without yet re-registering.
On the other hand, when a mobile station initiates a communication by transmitting an access probe to the network via the access channel in a given coverage area, the network may respond to that access probe by transmitting a response message to the mobile station on a paging channel in the same coverage area. For instance, to initiate a call when operating in a given coverage area, a mobile station may transmit on an access channel of that coverage area an access probe that defines a call origination request message. Upon receipt of that access probe, the network may then carry out various call setup functions and may transmit to the mobile station on a paging channel of the coverage area a channel-assignment message that directs the mobile station to use a particular air interface traffic channel to engage in the requested call, so that call may then begin.
If the coverage area on which the network receives a request message (e.g., call origination message) from a mobile station borders one or more other coverage areas as is typical, the network may transmit a response to the mobile station not only in the coverage area in which the mobile station transmitted the request but also in each adjacent coverage area, to account for the possibility that the mobile station has moved to another coverage area in the interim.
In a system where a wireless coverage area defines more than one paging channel, a mechanism is typically provided for selecting a paging channel to use for paging particular mobile stations, preferably in a manner that evenly or at least randomly distributes paging channel load among the various mobile stations the coverage area at any given time. One way to accomplish this is through use of a hashing algorithm keyed to mobile station identifier. The hashing algorithm may take the form of a probability function that, when applied to a given mobile station's identifier, will produce an index value that can be applied to a list of paging channels to indicate which paging channel should be used for that mobile station.
For instance, the hashing algorithm applied to the mobile station's identifier may produce a value from 0 to 1. Assuming, by way of example, that there are four sequentially numbered paging channels in the coverage area, a resulting value of 0 to 0.25 may then indicate that paging channel number 1 should be used, a resulting value of 0.26 to 0.50 may indicate that paging channel number 2 should be used, a resulting value of 0.51 to 0.75 may indicate that paging channel number 3 should be used, and a resulting value of 0.76 to 1.00 may indicate that paging channel number 4 should be used. Of course, the number of paging channels may differ from this, the manner in which the hash result would be applied to determine which paging channel to use for the mobile station at issue may differ as well, and the hash function itself may take any of a variety of forms.
When a network pages a mobile station in a given coverage area that has more than one paging channel, the base station may apply the established hashing algorithm keyed to the mobile station's identifier to determine which paging channel to use for transmission of the page message to the mobile station. Likewise, the mobile station may apply the same hashing algorithm keyed to the mobile station's identifier to determine which paging channel to monitor for receipt of a page message destined to the mobile station.
An overhead channel message (e.g., system-parameters message) that is broadcast in each coverage area may specify how many paging channels exist in the coverage area and may expressly or inherently define the paging channels to facilitate this process. Depending on the air interface protocol in use, for example, there may be a series of possible paging channels, and the overhead channel message may simply specify how many paging channels are defined for the coverage area, thus inherently indicating that the indicated number of paging channels beginning from the start of the sequence are in use.
In an example Code Division Multiple Access (CDMA) system, for instance, up to seven paging channels may be defined by spread spectrum modulation using, respectively, “Walsh codes” 1 through 7. If the overhead channel message indicates that the coverage area has just one paging channel, for instance, that would constitute an inherent indication that the coverage area has just paging channel #1, using Walsh code 1. Whereas, if the overhead channel message indicates that the coverage area has, say, three paging channels, then that would constitute an inherent indication that the coverage area has paging channels #1, #2, and #3, using Walsh codes 1, 2, and 3 respectively.
In this arrangement, or in analogous arrangements under other types of air interface protocols, one paging channel may be considered a “primary” paging channel, typically because at least that paging channel would be used in all (or substantially all) of the coverage areas in the system. Thus, each coverage area would employ the primary paging channel, and some or all of the coverage areas may also employ one or more additional paging channels. The primary paging channel may be defined with respect to a sequence of possible paging channels (e.g., as the first channel of the sequence or the last of the sequence), as in the CDMA example above for instance, or in any other manner.